PHP UML Generator

I strongly recommend BOUML which:

• is extremely fast (fastest UML tool ever created, check out benchmarks),
• has rock solid PHP import and export support (also supports C++, Java, Python)
• is multiplatform (Linux, Windows, other OSes),
• is full featured, impressively intensively developed (look at development history, it's hard to believe that such fast progress is possible).
• supports plugins, has modular architecture (this allows user contributions, looks like BOUML community is forming up)